Turbulent stratified entrainment and a new parameter for surface fluxes

Recent Res Devel Geophysics, (1999): 61-65 Turbulent stratified entrainment and a new parameter for surface fluxes Robert E Breidenthal Department of Aeronautics and Astronautics, University of Washington, Seattle, WA 98105-2004, USA ABSTRACT The entrainment hypothesis of Morton et al is generalized in a recent model for stratified turbulence The entrainment velocity is assumed to be always proportional to the ratio of the length and time scales of the entraining eddy The model further defines a new fundamental parameter, the vortex persistence, to be the number of times a vortex rotates during the time interval during which it moves a distance equal to its own diameter with respect to the interface In other words, the persistence parameter is pi times the ratio of the rotational to the translational speed of the eddy A measure of vortex stationarity, this parameter plays as large a role in stratified entrainment as the Richardson number Ri Near its critical value, even a small change in the persistence yields a large change in the entrainment rate at large Ri, as confirmed by experiment The same concept is applicable if the interface is a solid wall Thus wall fluxes may be laminar, i.e independent of the fine-scale turbulence, or turbulent, depending solely on the vortex persistence INTRODUCTION Geophysical flows are characterized by their large Reynolds numbers and by their stratification The former implies that such flows are commonly turbulent The latter introduces the reduced acceleration of gravity g’ From elementary dimensional analysis, the entrainment hypothesis of Morton et al[1] asserts that the entrainment velocity into a turbulent flow must be simply a constant of proportionality times the characteristic mean velocity there However, when some dimensional parameter enters the problem, it is possible to achieve a quantity of dimension velocity incorporating this parameter The entrainment hypothesis is no longer valid For example, in supersonic flow, the speed of sound a enters the problem, so that the entrainment velocity normalized by the characteristic turbulent velocity becomes a function of the Mach number [2,3] Likewise, in a stratified flow, g’ enters the problem, so that the dimensionless entrainment velocity is a function of the Richardson number The buoyancy acceleration g’ is based on the normalized density difference  across an interface in a gravitational acceleration g It is desirable to extend the entrainment hypothesis, so that it is always valid, even when the flow is stratified Stratified entrainment controls the vertical transport and mixing of scalars and momentum in the ocean and atmosphere In spite of its importance in geophysics, remarkably little is known about it In a comprehensive review, Fernando [4] concluded that the normalized entrainment rates measured in various laboratory flows differed widely In some flows, the entrainment rate even depended on the diffusivity of the stratifying agent [5] Usually, the entrainment velocity w e, when normalized by the characteristic impingement velocity w 1, was found to be proportional to the Richardson number raised to some entrainment exponent , w e/w1 = const Ri where the Richardson number is Ri = g’/w12, and is the characteristic size of the turbulence respectively Measured values of  vary widely, with typical values of -1/2, -1, and -3/2 This suggested some kind of pattern, with different entrainment regimes, but at the time of Fernando’s review, no general theory of stratified entrainment was available PERSISTENCE MODEL OF STRATIFIED ENTRAINMENT Since then, a general theory for stratified entrainment has been developed [6,7] In it, the entrainment hypothesis of Morton et al is generalized to include even those cases where g’ is not negligible Consider a vertical jet impinging on a thin, stratified interface as sketched in figure Cotel et al [8] showed that entrainment is achieved by a lateral vortex at the side of the impingement dome The original entrainment hypothesis of Morton et al would take the entrainment velocity w e to be proportional to the impinging velocity w 1, which is in turn proportional to ratio of the length and time scales of the impinging eddy of the jet However, the new model always takes we to be proportional to the ratio of the length and time scales of the entraining eddy, in this case the lateral vortex The generalized entrainment hypothesis seems to work for every entrainment regime, as discussed below, as well as in accelerating or compressible turbulence [9,3] The theory also proposes a new fundamental parameter The entrainment rate depends not only on the conventional dimensionless parameters, the Richardson, Reynolds, and Schmidt or Prandtl numbers, but also on the vortex persistence A measure of vortex stationarity, the persistence T is defined to be the number of rotations a vortex makes during the time it moves (with respect to the interface) a distance equal to its own diameter Thus it is the ratio of the rotation speed U2 to times the translation speed U1 of the large vortex with respect to a nearby interface (see figure 1) U1 U2 Figure Vortex persistence T =U2 / U1 The Reynolds number is Re = w 1 where  is the kinematic viscosity The Schmidt or Prandtl number is the ratio of the diffusivities of momentum to mass D or heat DT, respectively Sc =  D Pr =  DT The model supposes that these four parameters determine which of several different entrainment regimes corresponds to these conditions In each regime, the entrainment velocity is the ratio of characteristic length and time of the entraining eddy or process, according to the generalized entrainment hypothesis discussed above Figure is an entrainment diagram of the model for the nonpersistent limit, where T is less than some critical value Tcr, of order unity For simplicity, the Schmidt number is assumed to be greater than unity, and the interface is thin In each regime, the normalized entrainment rate is given, to within a dimensionless coefficient A single line between regimes indicates a continuous transition, while a double line indicates a discontinuous transition For example, if Ri > Re1/4, the entrainment rate is independent of Ri and proportional to Re-1/4 This is the smooth “flat interface” regime, where the stratification is so great that even the smallest turbulent eddies have insufficient kinetic energy to engulf a tongue of fluid across the interface In other words, the eddy Richardson number at the Kolmogorov microscale there is greater than one All fluxes here must be diffusive, since Roshko’s entrainment tongues are absent [10] Figure is the corresponding entrainment diagram for the persistent limit, T > Tcr Note that the entrainment rate for the flat interface regime now depends on the square root of the Reynolds number, a laminar flux, even though the flow is turbulent at large Re Sometimes turbulent flows generate laminar fluxes, depending on the vortex persistence The theory is in accord with about 80% of the experiments in the literature, including the peculiar diffusivity effects discovered by Turner [5] It is not clear why the theory fails to explain the other experiments Perhaps its most impressive demonstration was its successful prediction  of the entrainment rate for a tilted jet impinging on a stratified interface, an  experiment suggested by L. Redekopp.  When a vertical jet is tilted just 15  degrees and rapidly precessed about a vertical axis, so as to lower its  persistence, the entrainment rate across the interface suddenly decreased by  two orders of magnitude at a Richardson number of 10 [8].  There are two  points worth noting here.  The sign of the change is counter­intuitive, and all  conventional parameters were held constant.  In order to determine the  entrainment rate, the persistence must be considered Another example is the comparison between the impinging vertical jet and the impinging plume The value of  for the jet is –1/2 [8], while that of the plume is –3/2 [11] The two flows superficially resemble each other, but the plume is known to oscillate more from side to side than the jet [12] Pure ambient fluid reaches the axis of the plume, but not that of the jet Evidently, only a small change in nonsteadiness near its critical value is sufficient to fundamentally alter the physics In typical geophysical situations, one might expect the flow to be nonpersistent However, topographic features acting as fixed boundary conditions may impose a steadiness onto the flow so that it is persistent This is an open question at the moment INTERFACE AS A SOLID WALL If the concept of vortex persistence is valid for an interface that is stratified, it may also be valid for an interface that is solid, i.e a wall The boundary layer heat transfer coefficient has been measured on a flat plate in a wind tunnel as a function of the freestream turbulence [13] To their surprise, Edwards and Furber found that as long as the boundary layer itself remained laminar, the heat flux was independent of freestream turbulence In other words, the flux was completely independent of the presence of small-scale turbulence Of course, if the boundary layer itself becomes turbulent, the eddies within it are sufficiently strong and moving rapidly past the wall so that they are nonpersistent Compared to the persistent case, the flux is now much larger, since it depends on the presence of the small-scale turbulence and its corresponding small time scale According to Reynolds’ analogy, the heat transfer coefficient is proportional to the skin friction coefficient This raises the interesting possibility that skin friction may be reduced by deliberately introducing a strong vortex into the flow and holding it stationary [14,15] APPLICATIONS TO ROTATING FLOWS Even in a constant density flow, the high-speed fluid prefers the outside of a turn [16] Bradshaw [17] showed a direct analogy between stratified and rotating flows, deriving a quantitative relationship between the two based on a linear theory As a consequence, it is straightforward to apply any theory for stratified entrainment to rotating flows such as a tornado [18,19] or a wing tip vortex The tip vortex emanating from a lifting wing is known to grow at a remarkably slow rate in the near field, a problem of some interest in aviation safety It is generally recognized that an isolated tip vortex grows as if it were laminar, even though the Reynolds number is large and the flow appears turbulent [20,21] This can be explained by persistence theory [22] Near the radius at which the azimuthal velocity profile of the vortex is a maximum, the effective Richardson number in Bradshaw’s analogy becomes arbitrarily large The flow is so highly stratified there that the surface at this radius is “flat” Because an isolated vortex is not disturbed by any neighbors, the flow is persistent with respect to this surface Transport across this surface can only be diffusive, so that the momentum flux there is laminar The vortex grows slowly CONCLUSIONS The entrainment hypothesis of Morton et al is generalized to include stratification by assuming that the entrainment velocity is always proportional to the ratio of the length and time scales of the entraining eddy This is also consistent with observations in accelerating and compressible turbulence There is a second, subtler effect of stratification It introduces at least one surface into the problem, an isopycnal, from which the relative velocity of a vortex can be defined Thus an eddy velocity ratio, the rotational velocity of a vortex normalized by its translational speed with respect to the surface, becomes another independent parameter in principle The vortex persistence is proposed to play as large a role as the Richardson number Near its critical value, of order unity, only a small change in the persistence at high Richardson number can change the entrainment rate by orders of magnitude Turbulence generates surface fluxes at nearby interfaces that are laminar in some cases and turbulent in others, depending on the value of the vortex persistence If the persistence exceeds its critical value, the surface flux is laminar, i.e completely independent of the presence of small-scale turbulence The wall heat transfer coefficient can therefore have a laminar value even in the proximity of freestream turbulence As in the case of a stratified interface at large Ri, the fluxes at a wall are very sensitive to the persistence at large Re The theory seems to be accord with most, but not all, of the experimental observations Experiments are underway to test the model further in both stratified and wall flows REFERENCES [1] Morton, B.R., Taylor, G.I., and Turner, J.S 1956, Proc Roy Soc A, 2334, [2] Papamoschou, D and Roshko, A 1988, J Fluid Mech., 197, 453 [3] Breidenthal, R.E 1992, AIAA J., 30(1), 101 [4] Fernando, H.J.S 1991, Ann Rev Fluid Mech., 23, 455 [5] Turner, J.S 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Ann Rev Fluid Mech., 30, 107 [21] Jakob, J., Savas, O., and Liepmann, D 1995, AIAA J., 35(2), 275 [22] Cotel, A.J and Breidenthal, R.E 1999, submitted to Phys Fluids       Ri-3/2  Sc-1/2Ri-1     Re Sc-2/3Re-1/4   1      Sc-2/3Re-1/2   1                         Sc   Ri Figure Entrainment diagram, T < Tcr Ri-1/2     Re Sc-2/3Re-1/2   1   1 Ri Figure Entrainment diagram, T > Tcr ... between stratified and rotating flows, deriving a quantitative relationship between the two based on a linear theory As a consequence, it is straightforward to apply any theory for stratified entrainment. .. general theory of stratified entrainment was available PERSISTENCE MODEL OF STRATIFIED ENTRAINMENT Since then, a general theory for stratified entrainment has been developed [6,7] In it, the entrainment. .. is always valid, even when the flow is stratified Stratified entrainment controls the vertical transport and mixing of scalars and momentum in the ocean and atmosphere In spite of its importance